The goal of the research is to learn how cellular recognition, inductive interactions among developing neurons, and controlled timing of developmental processes all contribute to the formation of specific functional pathways in the vertebrate central nervous system. The studies are focused on the very early development of a small set of identified reticulospinal neurons, especially the Mauthner (M) neuron, their sensory inputs, and motor outputs in embryos of the zebrafish. (1) We propose to anatomically characterize the detailed developmental sequence of origin of sensory synaptic inputs to a single dendrite, the M-lateral dendrite, by labeling the developing cells with horseradish peroxidase (HRP), and to experimentally deprive the dendrite of the earliest of these inputs before the contacts are made. This will show how the axo-dendritic interactions that occur during synaptogenesis affect the observed patterning of inputs on the dendrite. (2) We propose to characterize by thymidine and HRP labeling, and by intracellular recording from known types of developing neurons in the spinal cord, how a single central axon, the M-axon, selects its synaptic targets. By relating when and where different kinds of spinal neurons develop and the growth of the M-axon through the cord we will learn if specific choices between possible target cells are required to explain the pattern of synaptic outputs of the M-cell, or alternatively, if the axon synapses with all of the neurons present along its pathway of growth. (3) Finally, we propose to obtain a library of monoclonal antibodies that specifically recognize zebrafish reticulospinal neurons, with the eventual goal of using these as probes to understand the molecular bases of specific cellular interactions during synaptogenesis.